1. Field of the Invention
The present invention relates to a piezoelectric thin film device including a single or a plurality of film bulk acoustic resonators (FBAR).
2. Description of the Background Art
FIG. 59 is an oblique view showing a schematic configuration of a main part of a conventional film bulk acoustic resonator 9.
As shown in FIG. 59, the film bulk acoustic resonator 9 includes a piezoelectric thin film 91, and an upper electrode 92 and a lower electrode 93 formed on respective main surfaces of the piezoelectric thin film 91. In the film bulk acoustic resonator 9, a square drive section 921 of the upper electrode 92 and a square drive section 931 of the lower electrode 93 are opposed to each other with the piezoelectric thin film 91 interposed therebetween, and when an excitation signal is applied to pads 923 and 933 electrically connected to the drive sections 921 and 931, an electric field is generated to excite vibrations inside the piezoelectric thin film 91 in an excitation region 911 where the drive sections 921 and 931 are opposed to each other. It is to be noted that the shapes of the drive sections 921 and 931 may not be square but may alternatively be circular.
In such a film bulk acoustic resonator 9, in order to prevent leakage of the excited vibrations from the excitation region 911 leading to generation of sub-resonance due to the outline of the piezoelectric thin film 91, an energy trapping structure is often adopted in which a cutoff frequency of acoustic waves is displaced by a means for partially changing a film thickness of the piezoelectric thin film 91, to prevent leakage of vibrations from the excitation region 911.
It should be noted that Japanese Patent Application Laid-Open No. 8 (1996)-242026 is a prior art document on the conventional film bulk acoustic resonator.
However, with the conventional film bulk acoustic resonator, there has been a problem in that in the case of using a piezoelectric material having a large electromechanical coupling factor, such as lithium niobate or lithium tantalite, a sufficient energy trapping effect cannot be obtained, and hence a frequency impedance characteristic becomes susceptible to spuriousness.